Aluminium alloy members made of heat-treatable aluminium alloys are used in a number of applications involving relatively high strength, high toughness and corrosion resistance such as aircraft fuselages, vehicular members and other applications.
To manufacture an aluminium alloy member, for example a sheet or plate, aluminium alloy is either direct chill cast as ingots or continuous cast in the form of a thick strip material, and then hot rolled and/or cold rolled to the desired thickness. The member then undergoes solution heat treatment. Solution heat treatment involves heating the metal to a suitably high temperature (e.g. 450-580° C.) to cause dissolution into solid solution of all the soluble alloying constituents that precipitated from the parent metal during hot and/or cold rolling. To retain these constituents in solid solution, the metal is rapidly quenched to ambient temperature to create a solid supersaturated solution. Usually, the metal is then aged or precipitation hardened by holding the metal at room temperature, or at a higher temperature to accelerate the effect, for a period of time to cause the spontaneous formation of fine precipitates through the diffusion of atoms in the supersaturated solid solution, whereby they form fine clusters or “zones”.
It is further known that the properties of an aluminium alloy member may be further improved by subjecting the member to a further heat treatment after quenching. During this so called “pre-ageing” heat treatment, some of the atoms in the supersaturated solid solution come out of the lattice structure and form seeds for the formation of fine clusters. This serves to stabilize the microstructure.
U.S. Pat. No. 3,135,633 discloses such a pre-ageing or “stabilizing” heat treatment to improve the mechanical properties of aluminium-magnesium-silicon alloys. In this process, wrought alloyed products are moved continuously through a first furnace to put the relevant alloying elements into solid solution, then through a quenching chamber and into a second furnace to be subjected to a stabilizing pre-ageing treatment. It is mentioned that the time interval between quenching and preliminary ageing should be less than ten minutes. To allow for rapid heating-up of the alloy, starting a few minutes after quenching, the second furnace is heated by forced hot air circulation.
A pre-ageing heat treatment is further described in EP-0805879-B1. In the disclosed method the metal is heated directly to a peak temperature in the range of 100 to 300° C., preferably in the range of 130 to 270° C., is maintained at the peak temperature for a very short dwell time and is then cooled directly to below a defined final temperature. This treatment is therefore also referred to in the art as temperature “spiking”, since the profile of the temperature versus time graph for such a process resembles a generally triangular, pointed, or slightly blunted spike. The treatment is reported to improve the ductility of alloys of a AA6xxx-series in the T4 temper while maximizing the paint bake response.
Another process involving a pre-ageing heat treatment is disclosed in EP-0480402-A1. The known process involves quenching an aluminium alloy sheet after solution heat treatment, allowing the sheet to hold still at room temperature for less than 60 minutes, and holding the sheet at a temperature of 50-150° C. for a period of from 10-500 minutes.
EP-0679199-A1 also discloses a pre-ageing or pre-tempering step at a temperature of 70 to 150° C. in between the quenching after solution heat treatment and ageing steps.